A conventional magnetic recording disk typically contains servo patterns written in preselected regions (“servo sectors”) on the disk surface. The servo patterns provide position and tracking information during a readout process. For instance, each servo pattern may store information that enables a read head to determine its relative position on the disk surface. In addition, the servo pattern also may be configured to cause the read head to generate a position error signal when it reads the servo pattern. The position error signal is then fed back to a controller that controls the head's tracking.
The servo patterns are typically written on the magnetic recording disk during a disk-manufacturing process, e.g., in a clean room environment. There are various known techniques for writing the servo patterns on the disk. One common technique is to program a magnetic write head to write the servo patterns as the disk rotates beneath the head. This technique, although effective, sequentially writes the individual patterns onto the disk and thus can be overly time-consuming, e.g., taking up to six hours or more to complete.
Moreover, in many cases the servo patterns include marks that are intentionally misaligned with the disk's concentric recording tracks. For example, a servo pattern may include at least some “off-track” marks that are used to generate the position error signal. In these cases, the write head must be precisely positioned relative to the recording tracks, e.g., using laser interferometry or the like, to ensure that the head can write magnetic transitions at their expected off-track locations. Because of the strict tolerances required for positioning the write head while writing servo patterns, this servo-writing technique usually suffers the disadvantage of having to expend considerable time and resources setting up and configuring complex and expensive head-positioning instrumentation.
Another known technique for writing servo patterns utilizes magnetic printing. A printing master is constructed having a plurality of ferromagnetic “teeth” that are constructed and arranged to coincide with at least one servo pattern. According to this technique, a magnetic recording disk is first subjected to a strong magnetic field that substantially saturates the disk along an initialization direction. The initializing field is removed and then the printing master is positioned so that its teeth are placed in contact with the disk surface at one or more servo sectors. After positioning the printing master, the pattern of the ferromagnetic teeth is transferred to the disk surface by applying a writing field in a direction that is substantially opposite to the initialization direction. The teeth may be used either to shield their contacted disk surfaces from the applied writing field (e.g., longitudinal recording), or alternatively to concentrate magnetic flux at their contacted surfaces (e.g., perpendicular recording). In either case, a copy of the teeth pattern is written onto the disk after the printing master is removed.
Contact printing for longitudinal media is generally described in more detail in U.S. Pat. No. 6,813,106, entitled Premagnetization Process for Printing Longitudinal Media, by Michael Mallary, issued Nov. 2, 2004; contact printing for perpendicular media is generally described in more detail in U.S. Patent Application Publication No. US 2003/0082395, entitled Master for Printing Servo Patterns, by Michael Mallary, published May 1, 2003.
The above-noted servo-writing technique provides various advantages over conventional head-based servo writers, especially with respect to the speed with which the servo patterns are written. However, during the magnetic printing process, stray fields between the ferromagnetic teeth often limit their ability to write high-frequency servo patterns. That is, if the ferromagnetic teeth are spaced too close together, they may not be able to effectively shield or concentrate these stray interstitial fields. Accordingly, the ferromagnetic teeth are limited to transferring magnetic mark sizes above a minimum feature size, e.g., on the order of hundreds of nanometers, before the undesirable effects of the stray fields become prohibitive.
Yet another known technique for writing servo patterns employs laser light to transfer the patterns to disk using heat assisted magnetic recording (HAMR). Like the other techniques, the disk is initially saturated substantially along an initialization direction. Thereafter, incident light is focused at selected areas on the disk surface to produce a pattern of heated areas corresponding to the servo patterns, and an external magnetic field is applied to the heated areas in a direction that is substantially opposite to the initialization direction. The strength of the applied field is selected to be greater than the coercivity of the heated (irradiated) areas of the disk media, yet less than the coercivity of the unheated media. As such, the applied field only reverses those magnetic domains in the heated regions, thereby copying the servo patterns to the disk. One example of HAMR-based servo-writing is described in more detail in U.S. Pat. No. 6,879,458, entitled Method for Thermally Writing Servo Patterns on Magnetic Media, by Sacks et al., issued Apr. 12, 2005.
While the minimum feature size that can be written using HAMR-based approaches is typically limited by the diffraction-limited spot size of the incident light, the resolution may be improved by utilizing an optical shadow mask having a pattern of holes constructed and arranged to coincide with at least one servo pattern. The shadow mask may be placed in close proximity to the surface of the disk and aligned with one or more of the disk's servo sectors. When the incident laser light is applied to the mask, the light passes through the holes and irradiates the exposed areas of the disk surface. For small hole sizes, there exists a trade-off between reducing the hole sizes, e.g., below the diffraction-limited spot size, and increasing the power of the incident light to heat the exposed disk media. As a result of this trade-off, the minimum feature sizes that can be written to disk through the optical mask is typically limited, e.g., to magnetic domains on the order of hundreds of nanometers.
For high-density servo patterns, smaller feature sizes are generally desired. Accordingly, there is a need for an improved technique for writing servo patterns on magnetic recording media. The technique should enable faster and less complex writing processes as compared with conventional head-based servo writers. In addition, the technique should be less limited in its minimum feature sizes as compared with current optical and contact servo-writing techniques.